How neutrinos could ensure a submarine’s nuclear fuel isn’t weaponized

Nuclear submarines might provide rogue nations with a path to nuclear weapons. But neutrinos could help reveal attempts to go from boats to bombs.

Neutrinos, lightweight subatomic particles that are released from the reactors that power nuclear subs, could expose the alteration or removal of the nuclear fuel for nefarious purposes, physicists report in a paper accepted in Physical Review Letters. Crucially, this monitoring could be done remotely, while a submarine is in a port with its reactor shut off.

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To ensure that countries without nuclear weapons don’t develop them, international inspectors monitor the use of many types of nuclear technology around the world. Nuclear submarines are particularly worrisome. Many use highly enriched uranium, a potent type of fuel that can be weaponized relatively easily. But submarines are protected from monitoring by a loophole. Unlike nuclear power plants, nuclear submarines are used for secretive military purposes, so physical inspections could infringe on a country’s national security.

“Neutrino-based methods can considerably reduce the intrusiveness by making measurements at a distance, without having to physically access the vessel,” says nuclear scientist Igor Jovanovic of the University of Michigan in Ann Arbor, who was not involved with the research.

These particles — specifically their antimatter variety, antineutrinos — stream in droves from operating nuclear reactors. The particles interact feebly with other matter, allowing them to pass through solid material, including a submarine hull. So a neutrino detector placed near a submarine could reveal what’s going on inside, say neutrino physicists Bernadette Cogswell and Patrick Huber of the Center for Neutrino Physics at Virginia Tech in Blacksburg.

Scientists have previously suggested using neutrinos to detect other nuclear misdeeds, such as nuclear weapons tests (SN: 8/20/18).

But submarines, often on the move, are hard to monitor with stationary instruments. When the vessels do sit in port, their nuclear reactors may be turned off. So the researchers came up with a solution: They’d look at neutrinos produced by the decays of varieties of chemical elements, or isotopes, that remain after a reactor shuts down. A detector located in the water about 5 meters underneath the sub’s reactor could measure neutrinos produced in decays of certain cerium and ruthenium isotopes. Those measurements would reveal if nuclear material had been removed or swapped out.

This method of monitoring a reactor that’s off is “very clever,” says physicist Ferenc Dalnoki-Veress of the Middlebury Institute of International Studies at Monterey in California.

But the idea would still require buy-in from each country to agree to detectors in submarine berths. “Something like this would be so much better if it wouldn’t require cooperation,” says physicist Giorgio Gratta of Stanford University.

Submarine monitoring may become more pressing in the near future. So far, all countries that have nuclear submarines already possess nuclear weapons, so the issue was hypothetical. But that’s set to change. The United States and the United Kingdom, two nuclear weapons states, announced last September that they are entering into a cooperative security agreement with Australia and will help the country, a non-nuclear weapons state, acquire nuclear submarines.

There’s little suspicion that Australia would use these submarines as a cover for a nuclear weapons program. But “you still have to worry about the precedent that that sets,” Cogswell says. So, she says, monitoring nuclear submarines is newly important. “The question was how the heck to do that.”